Silicon finds wide application in electronic device production. One particular utility is thin film transistors (TFTs). Such transistors are critical components in numerous devices. A device of great interest currently is a liquid crystal display (LCD).
It is a goal of the silicon device industry, and especially active matrix liquid crystal device (AMLCD) production, to achieve higher performance by replacing amorphous silicon (a-Si) thin film transistors (TFTs) with a polycrystalline silicon (poly-Si) version. This change would result in higher on-currents, better frequency response, larger displays, and potentially longer life-times. It would be possible to implement peripheral circuitry, as well as pixel switches, on the glass substrate. This would greatly reduce the number of external connections to the substrate, thus increasing reliability and reducing cost.
Poly-Si TFTs are very attractive for the above reasons. The poly-Si film required for TFT fabrication can be formed directly on a flat glass substrate with a high strain point. Such glasses have been proposed, for example, in U.S. Pat. Nos. 5,116,787 and 5,116,789 (Dumbaugh, Jr. et al.).
Alternatively, a noncrystalline film, or a mixed-phase film, can be deposited and then heat treated to form a polycrystalline film. M. K. Hatalis et al. explain in "Low Temperature Polysilicon Thin Film Transistors on Corning Code 1734 and 1735 Glass Substrates", Proceedings of the Society for Information Display, 1993, pp 724-7, the process for depositing a "mixed-phase" film. The deposited silicon film consists of a mixture of noncrystalline and crystalline regions, with the material becoming completely polycrystalline during a subsequent heat treatment. This procedure is preferred because it produces a film with a larger grain size for higher electron mobility and a smoother surface for subsequent gate dielectric deposition.
Again, a glass exhibiting a high strain point is demanded as is discussed by Hatalis et al., supra. If, however, the glass is compacted via a combined non-damage anneal/crystallization process, then glasses demonstrating somewhat lower strain points, such as Corning Code 7059 glass with a nominal strain point of 593.degree. C., can be utilized. Hatalis et al. reported crystallizing a mixed-phase film by a heat treatment of 12 hours at 550.degree. C. and crystallizing a noncrystalline film by a heat treatment of 36 hours at 550.degree. C. Accordingly, it can be appreciated that heat treating at a higher temperature, i.e., one no lower than about 10.degree. C. below the glass strain point, e.g., 580.degree.-590.degree. C., would result in shorter process times for concurrent compaction and crystallization. A relationship between time and temperature is shown in FIG. 1 of a paper by N. A. Blum and C. Feldman, "The Crystallization of Amorphous Silicon Films", Journal of Non-Crystalline Solids, 11, pp. 242-6 (1972), for silicon films deposited by electron beam deposition. Substrate temperature was 200.degree.-300.degree. C.
Liquid crystal display devices, whether passive or active, customarily embody thin, parallel, spaced glass panels with an intermediate liquid crystal layer.
A passive display device relies upon the threshold properties of the liquid crystal material. In an active device, the back panel, or active plane, has electronic switching devices, such as thin film transistors (TFTs). These are formed on the panel by photolithographic steps, together with attached circuitry. The front panel, or color plane, has transparent, colored dots, or stripes, in the case of a full color display.
Fabrication of the active plane, or active matrix, involves the use of multiple photolithographic steps which require precise alignment. This requires that the panels not only have precise dimensions as formed, but that such precise dimensions be retained during subsequent processing steps. However, these processing steps may involve thermal exposure at temperatures where a glass may undergo structural rearrangement and/or dimensional relaxation. Accordingly, it has become common practice to subject glass panels to a compaction process. This occurs after formation of the panel, and before further thermal processing during device formation.
Compaction involves reheating a glass body to a temperature below the glass softening point, but equal to or above the maximum temperature reached in a subsequent processing step. This achieves structural rearrangement and dimensional relaxation in the glass prior to, rather than during, the subsequent processing. It will be appreciated that the time held at a particular temperature will be based upon the time required for the particular processing step. Structural rearrangement and dimensional relaxation in a glass will take place more rapidly as the temperature is raised. It will be recognized that the time required therefore will be shorter as higher temperatures are utilized. Preliminary compaction is imperative where it is necessary to maintain precise alignment and/or flatness in a glass body during subsequent photolithographic processing. This is often the case in the manufacture of active panels for flat panel display devices.
It is economically attractive to compact glass sheets in stacks. However, this necessitates interleaving, or separating, adjacent sheets with a release material to avoid sticking. At the same time, it is necessary to maintain the sheets extremely flat, and with an optical-quality surface finish.
The panels used in an LCD device must, of course, be of optical quality. Strict cleanliness is a requirement during all processing. Any marring of the surface, such as surface scratches, indentations, or the like, must be avoided.
It has been proposed, in U.S. Pat. No. 5,073,181 (Foster et al.), to substitute a monolayer of submicron silica particles as a parting layer. However, this still requires removal, and, consequently, further handling.
Accordingly, a pending application discloses applying a permanent barrier layer film that performs the dual function of a parting layer during compaction. The film is colorless and transparent, is deposited from an atmosphere of an atomized, or ionized, refractory material, or reactive precursor, and is 50-500 nm thick. The application is Ser. No. 08/132,554, a continuation-in-part of Ser. No. 07/853,587, filed Mar. 18, 1992, in the name of F. P. Fehlner, and assigned to the same assignee as the present application.
Glasses displaying strain points as low as about 560.degree. C. are operable in the present invention. However, the temperatures to which LCD substrates are exposed have been continually increasing in recent years. Therefore, the preferred glasses will exhibit strain points greater than 560.degree. C. and, most preferably, in excess of 650.degree. C.
In order to consistently achieve the requisite optical quality in LCD panels, it would be desirable to avoid separate processing steps to the extent possible. In particular, it would be highly desirable to compact the glass substrate and crystallize a silicon film in one integrated operation. Any interfacial damage could then be annealed out and glasses with lower strain points employed.
It is then a basic purpose of the present invention to provide a method that incorporates these desired features. A specific purpose is to provide an improved method of producing a panel for an AMLCD device. Another specific purpose is to provide a novel means of producing a poly-Si film suitable for TFT fabrication on a compacted glass panel.